The present invention provides a method and apparatus for circulating a heat transfer fluid along a circulation path, by forced circulation, to cool a heat load. More particularly, the present invention relates to such a method and apparatus in which a cryogen, in preferably a subcooled liquid form, is converted into a saturated liquid through contact with a saturated gaseous form of the cryogen, a portion of the total available thermodynamic energy of either the saturated liquid, the saturated vapor, or both, is utilized to at least promote circulation of the heat transfer fluid and the heat transfer fluid is cooled by the saturated liquid and/or the saturated vapor.
The prior art has provided a variety of cooling methods and apparatus in which a cryogen, such as solid or liquid carbon dioxide, liquid nitrogen, etc., is utilized to cool a heat load and to promote circulation of a heat transfer fluid which can comprise evolved cryogenic vapor or a mixture of cryogenic vapor and air to and from a heat load. An example of such an apparatus is found in U.S. Pat. No. 3,163,022 in which the heat load comprises perishables contained within an insulated refrigerated compartment. An insulated refrigerant compartment is connected to the refrigerated compartment by supply and return conduits. The refrigerant compartment has a heat exchanger containing dry ice and a nozzle projecting from the heat exchanger into a venturi-type ejector provided within the supply conduit. The dry ice sublimates into a gas and the gas is expelled into the ejector of the supply conduit and then into the refrigerated compartment to cool the perishables. After having been heated through the cooling of the perishables, the gas returns to the refrigerant compartment through the return conduit. The returning gas transfers heat to the dry ice through the heat exchanger and thereafter, mixes with the sublimated gas in the ejector. The ejector produces a low pressure region to draw the returning gas from the refrigerated compartment and past the heat exchanger in the refrigerant compartment. Thus, a portion of the total available thermodynamic energy of the sublimated gas, that is a sum of its enthalpy and its kinetic energy, is being made to perform work in forcing the circulation of the sublimated gas between the refrigerant and refrigerated chambers. At the same time, the cooling potential of the sublimated gas is being used to cool the perishables.
The amount of cooling and circulation are coupled due to the self-pressurizing aspect of the apparatus. Therefore, the degree of cooling and the amount of circulation are necessarily limited. As will be discussed, the present invention provides a cooling method and apparatus in which the cooling supplied and the circulation of the coolant can be independently controlled over a greater range of possible operating conditions than such a prior art device as disclosed in the '022 patent.